Combinatorial chemistry refers to techniques to collect, test, and store the resulting data for a material library containing tens, hundreds or even thousands of different materials or compounds. Combinatorial investigations require rapid screening techniques to test and evaluate variations of composition, structure and property within a material library. X-ray diffraction is one of the most suitable screening techniques because abundant information can be revealed from the diffraction pattern, and the technique is fast and non-destructive.
Combinatorial screening with X-ray diffraction may be performed in reflection mode. In reflection mode, both an X-ray source and an X-ray detector are located to the same side of a sample being examined. For many samples or sample libraries, only one side of the sample is exposed and, therefore, X-ray diffraction can only be done in reflection mode. In some other cases, although two opposing sides of a sample are exposed, the thickness of the sample is beyond the penetration capability of the X-ray energy from the source and, again, only reflection mode diffraction is possible.
Many combinatorial chemistry applications require X-ray diffraction screening in the low Bragg angle range, such as the search for catalysts and new drugs. In the low angle diffraction measurement, the incident X-ray beam is spread over the sample surface into an area much larger than the size of the original X-ray beam. In combinatorial screening applications, sample cells are located close each other. Therefore, the spread beam may cause cross contamination in the collected diffraction data. There are two ways to overcome these problems. One way is to use a knife-edge to limit the diffracted area. The details of such a knife-edge may be found in U.S. Pat. No. 6,718,008, the substance of which is incorporated herein by reference. Another way to avoid cross-contamination is to perform the X-ray diffraction screening in transmission mode.
In transmission mode X-ray diffraction measurement, an incident X-ray beam is directed toward one surface of the specimen, and diffracted X-rays are emitted from a surface on the opposite side of the specimen. The incident beam is typically perpendicular to the sample surface so that the irradiated area on the specimen is limited to a size comparable to the X-ray beam size, allowing the X-ray beam to remain concentrated en route to the intended measuring area. Since the X-ray beam is focused in a relatively small spot on an incident surface of the sample, the risk of cross contamination between sample locations is minimized. An X-ray diffraction system dedicated to transmission mode operation is disclosed in U.S. Pat. No. 6,859,520.
Most existing X-ray diffractometers for combinatorial screening are dedicated to operation in either reflection mode or transmission mode. The sample library in a combinatorial screening system is typically aligned in horizontal direction, and during the screening process the sample library retains its horizontal orientation. This is necessary for holding powder samples or liquid samples without spillage. Otherwise if, for example, reflection mode diffraction data was to be collected on a system configured for transmission mode analysis, the sample library would have to be rotated away from the horizontal position, and the powder or liquid samples could fall out of the sample containers and/or be cross-contaminated with other samples. The same would be true if a transmission mode system were to be used for reflection mode diffraction.